This invention relates generally to circuits for monitoring or testing the condition of a vehicle battery, and more particularly to circuits of the type employing one or more indicator lights which are intended to provide a visual indication of the condition of charge of the battery.
A number of such battery monitors have been proposed and produced over the years, and have met with moderate success. Many employed one or more incandescent bulbs which are arranged to become illuminated when either an over-voltage or other abnormal voltage condition occurred to the vehicle electrical system.
One such prior device is illustrated in U.S. Pat. No. 2,991,413 dated July 4, 1961. This patented construction employed multiple incandescent lamps or bulbs which become progressively illuminated to indicate the condition of the battery at any particular time. In one embodiment, three bulbs were provided, indicating low, medium and high voltage conditions. Referring to FIG. 1 in this patent, under low output conditions only the one bulb was illuminated. If the battery voltages reaches higher values, both the low and medium bulbs would be illuminated, and in the event that a high voltage condition occurred, all three bulbs would be lighted.
In another embodiment shown in FIG. 4 of the patent, additional circuitry was provided, involving the addition of two extra relays, such that when the bulb which indicated the high voltage condition was illuminated, both of the other bulbs were kept deenergized. Similarly, for a battery condition corresponding to a medium voltage, only the one bulb (55) was illuminated. The arrangement with the additional circuitry provided that only one lamp would be illuminated at any particular time.
While the above device operated more or less in a satisfactory manner, it can be readily appreciated that it was costly to produce, and also represented a solution of questionable reliability. In the embodiment described immediately above, a total of five relays was involved, leading to possible problems with arcing of contacts, contact sticking and deterioration; moreover, slight variations in magnetic field between different coils of the same type caused difficulty. In addition, it was found that such relays were generally very expensive to manufacture. While it may have been possible to obtain satisfactory performance of such a unit under ideal test conditions such as those which are found in a laboratory, the problems associated with mass-producing such items were considerable. In particular, there were variations in the physical and electrical characteristics of the coils associated with the relays, as well as variations in the springiness of the contacts. Such variations often led to erratic or unreliable operation, particularly over extended periods of use, or under adverse conditions as were typically found in the automobile environment. It can be readily appreciated that the equipment located in an automobile engine compartment is subjected to temperature extremes from well below 0.degree. F., to upwards of 250.degree.-300.degree. F. Accordingly, any sensitivity in the above circuit as far as temperature variations is concerned, would have deleterious effects on the operation and reliability of the device. Moreover, even under the best of circumstances, conventional relays are subject to failure when operated over an extended period of time, due to deterioration of the contacts associated therewith. Such deterioration can be the result of either arcing, or else normal oxidation which inevitably occurs, unless special steps are taken to prevent these occurrences. Such steps may be in the nature of gold plated contacts, etc., which tend to resist oxidation. However, even with gold contacts, arcing is frequently a problem.
Another arrangement is illustrated and described in U.S. Pat. No. 4,027,231 dated May 31, 1977. The circuit shown therein involved a Zener reference diode, and three current-amplifier transistors which are connected respectively to light-emitting diodes. This particular patented circuit operated in such a way as to indicate the percentage of useful battery life remaining, by providing for illumination of one, two or all three light-emitting diodes. However, no provision was made for selectively energizing the light-emitting diodes to indicate either a low, normal, or else a high battery-voltage condition. Accordingly, there was the possibility of confusion arising unless the user who was employing the device understood its operation, and could make a somewhat subjective determination as to what the successive stages of illumination meant.
Still another arrangement is disclosed in U.S. Pat. No. 3,810,144 dated May 7, 1974. This battery monitor was in the form of a relatively simple circuit, employing one Zener diode and two transistors, as well as in incandescent bulb. In the particular unit illustrated, one transistor was normally on, with the second being normally off. When a drop in battery voltage occurred, falling below a pre-set value, the transistor switched their respective states, thus energizing the indicator light and providing an indication that low battery voltage was being experienced.
Yet another construction is illustrated in U.S. Pat. No. 1,338,170, wherein there is disclosed a circuit for monitoring the condition of the individual cells in a three-cell battery. Use is made of three incandescent bulbs, and the arrangement is such that when one particular cell fails, the one bulb corresponding to that cell will become extinguished. Accordingly, an indication as to the condition of each of the particular cells can thus be readily obtained.
U.S. Pat. No. 4,025,916 dated May 24, 1977 relates to a rather involved circuit for determining the condition of a battery. In particular, the battery is subjected to a load, and thereafter the drop in voltage which occurs after a pre-determined time interval has elapsed, is employed to indicate the condition of the battery. The battery voltage, following the elapsed time and with the battery under load, is compared to a reference voltage, and the difference is used to gate one of three indicator circuits corresponding to either adequate, marginal, or else poor output voltage. Presumably this circuit operated in an acceptable manner, but as can be readily determined from a perusal of the drawings, the circuitry involved was quite complicated. Under such circumstances, it is believed that such a circuit would be relatively expensive to manufacture and build.
Yet another monitor arrangement is illustrated in U.S. Pat. No. 4,247,813. FIG. 4 of this patent shows a two-terminal circuit which is intended to be connected to the battery being monitored, the circuit having a light-emitting diode and being arranged in such a manner that the diode becomes illuminated when either a high battery voltage or else a low battery voltage is sensed. With the circuit of FIG. 4, it could not be determined whether the abnormality was on the high side or the low side. An alternate arrangement is illustrated in FIG. 5 of the patent, wherein there are provided two light-emitting diodes, one of which is intended to be illuminated under conditions of high battery voltage and the other illuminated under conditions of low battery voltage.
While the above circuits operated in a somewhat satisfactory manner, several disadvantages became apparent. One or two of the circuits disclosed in the above patents were rather complex, and thus tended to be expensive to manufacture and produce. In addition, in U.S. Pat. No. 1,338,170 wherein the condition of each of the battery cells is monitored, it is necessary to make special connections to each cell. In the present day automotive industry such connections would be considered much too cumbersome to be of a practical value, and accordingly this circuit would not be suitable for use in today's vehicles.
In still others of the patents, it is often not clear whether the abnormality that is being sensed represents either a high voltage or a low voltage. In addition, in the absence of emitted light coming from a device having a single bulb, it is not clear to the operator if a normal battery condition was being monitored with the light merely being off, or if there existed the possibility of a malfunction in the circuit (such as a burned out bulb), in which case the device would not be operative. Such arrangements are generally to be avoided if at all possible, since an inoperative circuit which does not provide an indication of this condition can give the user a false sense of security.
In the above patent that involves multiple relays (five), the complexity of the device is too great to make its use practical in a modern vehicle. It can be readily appreciated that where hundreds of thousands of automobiles are manufactured every year, cost reduction is considered to be very important from the manufacturer's standpoint. This is true to such an extent that even reductions on the order of one cent or less are often considered to be worthwhile, as far as a particular system or component of an automobile is concerned. It must be realized, too, that many of the improvements that are introduced in a current model year will be carried over to future years, and may appear in manufacturing runs over periods of five or ten years, or more.